1. Field of the Invention
The present invention pertains to pyrotechnic bodies, preferably circular wafers and most preferably washer-shaped discs, made of conventional propellant or gas generant compositions which are fabricated to have a special or unique geometry, especially when stacked or arranged in side by side array in the combustion chamber of a gas generator or inflator. The inflator produces a gas which, after filtering out condensed phase products, is preferably used to inflate a gas bag which serves as a vehicle occupant cushion during a collision. More particularly this invention relates to a novel geometry for the generant body wherein a plurality of equally spaced projections which have special size, shape and location are integrally formed (molded) into one or both sides or faces of the body, preferably both, thereby facilitating spacing of plural bodies when stacked or assembled, for example, as an array and preferably side by side in abutting relationship in an inflator thereby enhancing combustion and gas flow during use.
Even though the generant bodies of this invention are especially designed and suited for creating gas for inflating passive restraint vehicle crash bags, it is to be understood that they would function well in other less severe inflation applications, such as aircraft slides, inflatable boats and inflatable lifesaving buoy devices; would more generally find utility any place a low temperature, non-toxic gas is needed, such as for a variety of pressurization and purging applications as, for example, in fuel and oxidizer tanks in rocket motors; and would be useful as a propellant for recoilless rifles and missile systems, particularly in in-tube rocket motors of the short-burning, high-mass type as disclosed in U.S. Pat. No. 3,886,006.
2. Description of the Prior Art
Automobile gas bag systems have been developed to protect the occupant of a vehicle, in the event of a collision, by rapidly inflating a cushion or bag between the vehicle occupant and the interior of the vehicle. The inflated gas bag absorbs the occupant's energy to provide a gradual, controlled deceleration, and provides a cushion to distribute body loads and keep the occupant from impacting the hard surfaces of the vehicle interior.
The use of such protective gas-inflated bags to cushion vehicle occupants in crash situations is now widely known and well documented.
The requirements of a gas generant suitable for use in an automobile gas bag device are very demanding. The gas generant must have a burning rate such that the gas bag is inflated rapidly (within approximately 150 milliseconds). The burning rate must not vary with long term storage (aging) or as a result of shock and vibration during normal deployment. The burning rate must also be relatively insensitive to changes in humidity and temperature. When pressed into wafers, cylinders, discs or whatever shape, the hardness and mechanical strength of the bodies must be adequate to withstand the mechanical environment to which they may be exposed over the expected inflator system lifetime of at least ten years without any fragmentation or change of surface area. Excessive breakage of the bodies could potentially lead to system failure where, for example, an undesirable high pressure condition might be created within the gas generator device, possibly resulting in rupture of the pressure housing.
The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which is easily filtered to remove solid and liquid combustion by-products, and thus preclude damage to the inflatable bag or to the occupant of the automobile.
The requirements as discussed in the preceding paragraphs limit the applicability of many otherwise suitable compositions and shapes thereof from being used in automotive air bag gas generators.
Inflators such as shown in commonly assigned U.S. Pat. Nos. 4,005,876; 4,296,084 and 4,547,342 contain gas generant in the form of pressed pellets or tablets (similar in shape to ALKA-SELTZER.RTM. buffered aspirin) which are randomly packed into the inflator combustion chamber. While such an arrangement may be satisfactory for some purposes, the tablets present a high initial surface area for burning and thus do not provide as soft of an inflation onset as may be desired and achieve low volumetric loading fraction due to the volume lost in the interstitial spaces between tablets.
To achieve slower inflation onset and increase volumetric loading fraction, it has been proposed in U.S. Pat. Nos. 3,901,530 and 4,131,300 to form the pyrotechnic combustible material for inflators in the form of separate discs arranged side by side with separator means disposed between adjacent discs to facilitate quick and uniform combustion of the material.
It has been proposed in U.S. Pat. No. 4,200,615 to fabricate a propellant grain from a plurality of washer-shaped discs 26 arrayed side by side on a linear igniter 8 in an inflator combustion chamber.
It has also been proposed in U.S. Pat. No. 4,158,696 to press the propellant into wafers and to configure each wafer in a manner to allow space for a propellant powder to be uniformly placed between the wafers to cushion each wafer from shock and vibration and to insure near instantaneous ignition of all wafers. The disposition of propellant powder between the wafers, however, may undesirably result in a fast onset of burning thus producing too high of an initial pressure peak as well as a decreased burn surface neutrality.
More recently it has been proposed in commonly assigned U.S. Pat. Nos. 4,890,860 and 4,998,751 to fabricate an inflator grain by assembling a plurality of washer-shaped propellant wafers or discs in alternating relationship with a plurality of similarly shaped meshed cushion members which are held in compression in the inflator combustion chamber to achieve improved performance.
It is also known from U.S. Pat. No. 4,817,828 that an inflator grain or material 60 may be formed by assembling a plurality of cylindrically shaped grains 64 and 66 (FIG. 5), each having an array of axially alignable central and outer peripheral gas passages. The end surfaces of the grains are provided with peripherally located projections 158 and 160, and 164 and 166 (FIGS. 5 and 8) which abut each other thereby acting as standoffs or spacers thus allowing radial gas flow between grains. The plurality of grains require a rather precise and complex assembling operation involving additional features and components to insure proper alignment of the standoff pads and gas passages. This alignment is achieved by placement of a multi-arm retainer device (FIG. 9) about the overall grain 60 which engages V-shaped notches 178 (FIG. 6) and 180 (FIG. 7) in the outer sides of each grain. The retainer becomes a permanent part of the inflator.
It is also known from U.S. Pat. Nos. 4,191,897 and 5,101,730 to provide washer-shaped gas generant discs or moldings with projecting knubs or islands on each side whereby a spacer zone would be created between stacked disc-shaped moldings thereby attaining improved combustion of the discs.